And the point goes to smodmog.
Chevrolet was developing an improved generation of air-cooled, horizontally opposed Corvair engines. It would be a modular system, with 4, 6, 8, and even 10-cylinder versions. Test 4- and 6-cylinder engines were built. To make a 10-cylinder engine, a 4 and a 6 were connected.
When I saw an air-cooled 10-cylinder, FWD, 1962 Impala, I knew it had to be a puzzle.
I don't have a reference guide to GM XP-series cars, but I have seen indications this car may have been called XP-790 or XP-787. XP-790 eventually became the Firebird show car, but it had no drivetrain.